1. Field of the Invention
The present invention relates to an implantable heart stimulator, e.g. a pacemaker or a defibrillator.
2. Description of the Prior Art
In conventional pacemaker technology often a single band-pass filter is used in the sensing circuit of the pacemaker in order to detect electrical heart signals. When using this known technique the origin of a signal that caused a sensed event is difficult to determine.
A ventricular event occurring early in the heart cycle (prior to a normally timed QRS-complex) and arising from a focus in the ventricles is often referred to as a premature ventricular contraction (PVC).
If a PVC not is detected due to undersensing it can cause inappropriately timed, asynchronous or competitive stimulation pulses to be delivered. Undersensing is defined as a failure of the pacemaker to sense an electrical signal related to a heart event, e.g. a PVC, due to the sensitivity of the sensing circuit of the pacemaker being set too low. This can often be corrected by programming the pacemaker to a more sensitive setting, i.e. decreasing the value of the sensitivity level.
U.S. Pat. No. 4,880,004 discloses an implantable cardiac stimulator for detecting and treating cardiac arrhythmias. The stimulator includes a sense amplifier responsive to sensed cardiac signals for detecting and distinguishing normal and abnormal cardiac activity within the sensed signals. The sense amplifier includes an automatic gain control amplifier, a filter and a comparator having a pair of signal channels for processing the sensed signals according to different frequency bandpass characteristics to establish sensing thresholds, margins and signal gain. One of the signal channels constitutes a feedback loop for determining the signal gain and the sensing margin for the other channel.
In U.S. Pat. No. 5,350,402 an atrial defibrillator is disclosed including a first detector for detecting R-waves of the heart and a second detector for detecting T-waves of the heart. The detection criterion is based on a predetermined time interval relationship between the R-wave and the T-wave. According to a software implementation of the T-wave detector, a microprocessor may be implemented for filtering the output of a sense amplifier with a high-pass filter and a low-pass filter. The derivative of the filtered signal is calculated by discrete differentiation of the filtered data and the derivative is re-filtered with a low-pass filter. These values are used in further calculations to determine if a T-wave is detected.
In U.S. Pat. No. 5,755,739 an adaptive and morphological system for discriminating P-Waves and R-waves inside the human body is disclosed. A drawback of a system using morphological recognition is that it probably is not fast enough for real time operation and that it is often implemented by a microprocessor that has unacceptably high energy consumption.
In U.S. Pat. No. 4,305,396 an improved automatically rate adaptive pacemaker is disclosed. The theory behind this patent is that a correlation has been identified between e.g. the amplitudes of the R-wave and T-wave and the heart rate. This correlation is then used to control a rate-responsive pacemaker. The peak values of the QRS-wave and T-wave, respectively, are detected in detection windows using conventional techniques. The detected values are then applied to a correlation block where a rate-controlling signal is generated.
European Application 0 917 887 discloses a cardiac event detecting system for an implantable heart stimulator intended to be connected to the heart of a patient via at least two unipolar electrode leads, or at least one bipolar electrode lead having one electrode pole in the atrium and one electrode pole in the ventricle, for sensing heart signals. This system has at least two signal channels for signals sensed between the two electrode poles and between one of the electrode poles and the stimulator capsule, respectively.
European Application 0 646 390 discloses a heart stimulator having an atrial electrode in an atrium of a heart and a ventricular electrode in a ventricle in the heart. In order to sense stimulated events in the heart a detector is connected between the atrial and ventricular electrodes to measure electrical heart signals between them.